An Ion Implanter consists in general of an ion source which ionizes atoms and molecules of solid, liquid, or gaseous feed materials, an electrostatic extraction and pre-acceleration field, an analyzing system where the ions are separated according to their masses, an acceleration system, and a scanning system to distribute the ions uniformly over the target. In addition, vacuum and control systems are necessary for operation.
The ion source typically employs a tungsten filament located within an arc chamber that has orifices for the introduction of gas or vapor atoms and a slit for the extraction of ions. The filament is directly heated by passing an electric current through it. This heating causes thermionic emission of electrons from the surface of the filament. An electric field, typically 30-150 volts, is applied between the filament and the arc chamber. This field accelerates the electrons from the filament area to the arc chamber walls. A magnetic field is introduced perpendicular to the electric field and causes the electrons to spiral outward increasing the path length and chances for collisions with the gas molecules. The collisions break apart many of the molecules and ionize the resultant atoms and molecules by knocking outer shell electrons out of place. As charged particles, these atomic or molecular ions can now be controlled by magnetic and/or electric fields. With one or more electrons missing, the particles carry a net positive charge. An extraction electrode placed in proximity to the slit and held at a negative potential will attract and accelerate the charged particles out of the arc chamber through the slit. An additional electrode biased positive follows the extraction electrode. This electrode is called the decal electrode and together the two perform beam extraction and initial focusing.
The beam, which at this point consists of many types of atoms and molecules, then passes through mass analysis. A properly shaped and oriented magnetic field will bend the path of each type particle a specific amount. Typically, the strength of this field is adjusted so that the desired type particle path is bent 90 degrees. Heavier particles will not make 90 degrees and lighter particles will do more than 90 degrees. Thus, the beam of all particles will be separated into a number of pure beams all following different paths. This system is called the mass analyzer.
Following the mass analyzer is a system of apertures, which allows the desired beam to pass, but blocks all other beams, and a beam shutter to gate the beam off and on at the appropriate times.
At this point, typically another system of electric fields are used to further accelerate and focus the beam. Acceleration potentials of zero to 160,000 volts are used, depending on the desired depth of deposition.
At this point, the beam is pure and sufficiently focused to be used in a system where the beam is fixed and the wafers are scanned in two planes through the beam. This scanning is done so that each area of the wafer is exposed to each area of the beam and nonuniformities in beam density are cancelled out, producing a uniform dose across the wafer.
Other types of systems fix the wafer position and electrostatically scan the beam. These systems require further focusing with an electrostatic lens system and set of X-Y scan plates to achieve the required uniformity.
The entire "beam-line" and ion source of all ion implanters must be maintained at a state of high vacuum during operation. When produced under normal conditions, ions are very short lived. They tend to acquire electrons from other atoms that they constantly come into contact with. Additionally, collisions with other gas molecules will alter the path much the same as a cue ball that strikes another ball along its path. In either case, the results are undesirable.
An expected failure mode within an ionization implanter is failure of the source or filament element. In common terms, the filament element burns out in the manner, in the way a light bulb element burns out during use. Where this "burn-out" failure occurs, the source assembly containing the filament must be removed from the implanter and a new source assembly installed. This process requires releasing the ion implanter vacuum, removing the old source or filament, installing a new filament, and then bringing the implanter back under vacuum and applying power to the filament. Further, restoring the filament requires a curing or "burn-in" time and an out-gassing procedure to release the gasses produced by the initial application of current to the new filament. During this time, as is well known in the art, impurities are driven from the new filament by the heating of the filament. These impurities are released into the machine in the form of a gas. In the process of filament "burn in" or curing, these gases are continuously removed until the cumulative effect of heating the filament to elevated temperatures drives off or causes the removal of the filament impurities. The whole process may take 45 minutes or longer. During this time, the ion implanter machine cannot be used for its intended purpose.